When a transmitter is required to transmit a signal which contains components at several distinct frequencies, nonlinearities in the final power amplifier give rise to intermodulation products (IMPs) at the output of the amplifier at frequencies other than those present in the input signal. The IMPs therefore distort the output and hence cause interference in adjacent channels. This effect gives rise to a significant technical problem where the frequency of the input signal to the final power amplifier stage represents information, for example if the input signal is the output of a parallel tone modem, or where the transmitter is being used to transmit several different signals at the same carrier frequency, as in an independent sideband mode of operation. For such Purposes, a typical specification requires that any IMPs should be reduced by at least -35dB relative to the wanted signals. Although it is possible to achieve this with amplifiers using valves, it has been difficult and impractical to produce IMPs reduced by more than -30dB with solid state class B linear amplifiers operating at radio frequencies.
Typically, attempts have been made to reduce distortion in such amplifiers by the use of inverted feedback techniques and a number of amplifier stages, in order to compensate for the reduction in gain produced by the negative feedback. Improvements may also be obtained by carefully selecting the biasing conditions of the transistors used. Such design considerations are well discussed in the literature. However, such techniques have been found ineffective in meeting the high specifications required.
Also, it has been proposed to use predistortion circuits interposed between a signal source and a transmission line so that the distortion imposed on the signal as it is transmitted along the line can be compensated allowing the signal to be correctly received.
U.K. Patent Specification No. 2204202 discloses a method of analysing the distorting IMPs produced in a linear Power amplifier and the use of a bank of predistortion circuits to compensate for such distortion. However, the techniques disclosed in this specification have practical limitations in that the analysis and predistortion are carried out as separate procedures, possibly in respect of different amplifiers having the same nominal specification. Furthermore, each predistortion circuit consists of a network of electrical components and is therefore applicable to a broad band of, as distinct from discrete, frequencies.
It is an object of the invention to provide an amplification system which at least alleviates the aforementioned problem.